The proposed study will accurately estimate the short-term variability of blood pressure in childhood, analyze the components of the variability, and assess the relevance of estimates of variability to current problems in childhood blood pressure research. For a population of 290 school children, nine to twelve years old, blood pressure will be measured at four school visits, and then at two home visits for the children, their siblings, and parents. These procedures will be repeated one year later. Variabiliy will be estimated for each individual child, and frequency distributions of variability will be obtained over the group of 290 children, from which a representative estimate of variability will be calculated for the average child. A recently-developed algorithm will be used to caculate the number of visits and measurements per visit needed to characterize blood pressure with a desired level of confidence. Familial correlations and tracking correlations will be calculated for the study participants, and the effects of the various components of blood pressure variability on these correlations will be assessed.